1. Field of the Invention
This invention relates to the processing of a hydrocarbon stream that is generally in the gasoline boiling range to remove sulfur therefrom. This invention also relates to enhancing at least a fraction of said stream by catalytic cracking while desulfurizing same.
2. Description of the Prior Art
Gasoline boiling range hydrocarbon streams are routinely generated by various processes in crude oil refineries or chemical plants. For example, a hydrocarbon stream containing gasoline boiling range hydrocarbons and other hydrocarbons outside the gasoline boiling range, e.g., a vacuum gas oil, is conventionally catalytically cracked in a refinery to produce gasoline boiling range material. Some chemical plants are designed to steam crack various liquid hydrocarbon feedstocks, such as straight run naphtha to produce light olefins (ethylene, propylene, butenes, etc.) and aromatics (benzene, toluene, xylenes, etc.). Steam cracking also produces an important by-product known as pyrolysis gasoline (“pygas”) which is generally within the gasoline boiling range. These various gasoline streams are ultimately blended with one another and/or other gasoline streams to form a finished, commercial gasoline product for sale to the public.
Due to environmental regulations, the level of total sulfur allowed in finished gasoline has been reduced and likely will be reduced more in the future. Accordingly, it is important to reduce the sulfur content of the various gasoline blending streams that go into formulating finished gasolines.
For example, pygas is used as a gasoline blending stream and is desirably desulfurized to some extent before the gasoline blending operation. The lower the sulfur content of such a blending stream the more valuable it is because of the low sulfur requirements for finished gasoline. This assumes that the octane is not significantly lowered from hydrodesulfurization. Currently, full range pygas from a cracking plant is first stabilized in a stand-alone, first-stage hydrotreater to remove reactive olefins. Thereafter, the pygas is fractionated (split) in a separate, upright splitter tower into a light pygas fraction and a heavy pygas fraction. The light fraction is desulfurized in a separate hydrodesulfurization (“HDS”) unit and then subjected to solvent extraction for the separate recovery of aromatics. The heavy fraction is sent to gasoline blending without HDS processing.
Due to ever tightening environmental regulations, it is desirable to desulfurize the heavy pygas before sending it to gasoline blending, but not necessarily to the same sulfur specifications as those set for the light pygas. Desulfurization of the heavy pygas could desirably reduce its sulfur content below the legal requirement or even well below such requirement thereby rendering that heavy pygas more valuable as a blending stock since it would meet or exceed sulfur requirements for the finished gasoline even before blending.
Heretofore, it has been taught to subject both light and heavy catalytically cracking gasoline fractions to HDS, see U.S. Pat. No. 6,334,948 (Didillon et al.). Didillon et al. contemplate the use of a distillation zone to form the light and heavy fractions with HDS zones outside or inside the distillation zone. To achieve their desired results, Didillon et al. require the use of a wholly nickel based catalyst on the light fraction and the use of a conventional HDS catalyst such as a Co/Mo based catalyst on the heavy fraction. Didillon et al. show by way of their Example 3 that when not using an entirely nickel based catalyst on the light fraction, but rather using a conventional HDS catalyst on both the light and the heavy fractions, their desired sulfur reduction results were not achieved as represented in their Tables 7–8. Further, Didillon et al. require mixing of the light and heavy fractions after subjecting each fraction to HDS. Accordingly, Didillon et al. not only teach, but require both HDS of the light fraction with a catalyst containing solely nickel, and mixing of the light and heavy fractions after HDS of each fraction. Additional teaching of related art was done by Johan W. Gosselink (“Sulfide Catalysts in Refineries”; CATTECH, Vol. 2, No. 2, December 1998 pp. 127–144). In FIG. 9 he teaches the use of a catalytic distillation tower/reactor to hydrodesulfurize, a lighter fraction with a CoMo/Al2O3 catalyst, and hydrodesulfurize, a heavier fraction using NiMo/Al2O3 beds. He teaches using a gas oil feed to produce a low sulfur diesel product by combining the two (light and heavy) desulfurized streams. This invention teaches away from this art by having a lighter gasoline range feed and separating the light and heavy product.